Longer term secondhand smoke exposure and chronic obstructive pulmonary disease (COPD) severity. (a) Locally weighted regression scatter plot smoother (LOWESS) procedure was used to graphically depict the relationship between 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL)-to-creatinine ratio and COPD severity score in a flexible manner (shown in orange). The blue line is the fitted linear regression line. The gray area shows the 95% CI. (b) LOWESS plot of COPD severity score residuals that adjust for age, sex, race, educational attainment, and smoking history, and NNAL-to-creatinine ratio (orange line). The blue line is the fitted linear regression line. The gray area shows the 95% CI.